1. Field of the Invention
The present invention relates to aircraft instruments, and more particularly to apparatus for the detection of windshear encounters during the takeoff and landing phases of flight utilizing the sensing of energy change and change of energy rate of the aircraft. 2. Description of the Prior Art
Low altitude encounters with a weather condition known as windshear have resulted in several aircraft accidents in the past. Windshear may be defined as a sudden change in the magnitude or direction of the prevailing wind, this being a difference between an airmass reference proximate to the aircraft and an inertial or ground-based reference.
In an encounter with a windshear, the aircraft may experience either an airspeed gain or loss dependent on the sign of the wind change. Similarly, it may experience a downward or upward wind sufficient to perturb the aircraft's flight path appreciably. The most dangerous of the windshear conditions are those resulting in airspeed loss or downward wind that tends to push the aircraft toward the ground. In the former case, loss of airspeed may result in a stall condition wherein the aircraft has lost lift If the pilot attempts to prevent the loss of speed, he will have to decrease the pitch attitude of the aircraft thereby causing the aircraft to descend toward the ground. In the latter case, the downward motion of the airmass can cause the aircraft to descend toward the ground and the pilot's attempts to arrest the descent by increasing the attitude of the aircraft can result in a stalled condition. Consequently, it is desirable to notify the pilot as quickly as possible of impending windshears so as to enable him to take corrective action as soon as possible.
A number of attempts and proposals have been made in the past to provide the pilot of an aircraft during a takeoff phase of landing approach with a warning of impending windshear conditions. Many of these have involved some means for detecting changes in ground speed as by using a ground based reference such as distance measuring equipment (DME) or by using an airspeed transducer and measuring the rate of change of its output, augmented by longitudinal acceleration to provide an inertial component related to the earth. Still other proposals have added vertical accelerometers to provide measures of aircraft vertical motion produced by the effects of windshear or utilized an input in accordance with the downdrift angle of the aircraft from the desired flight path. Some prior art windshear detection systems typically used a fixed, predetermined threshold of the rate of change of the wind with respect to time. More sophisticated systems, as for example the present assignee's U.S. Pat. No. 4,728,951, entitled "Vertical Windshear Detection for Aircraft", utilized a variable threshold dependent on the magnitude of the measured wind rate.
Prior art systems suffered either from the inability to detect low level sustained windshears due to high threshold levels required to prevent nuisance annunciations or complicated timing circuitry that added complexity to the system. The present invention overcomes these shortcomings by utilizing a direct integration of the change in the energy and energy rate of the aircraft. Hence, loss or gain of energy or energy rate is the criteria for windshear detection. If the loss or gain exceeds a predetermined threshold, annunciation of a windshear condition is presented to the pilot.